Earlier this month, CDC officials released a statement saying they expect AFM cases to decline as 2018 draws to a close. In recent years, most cases of AFM have been reported from August to October followed by a “marked reduction” in November, according to the statement.

The illness is truly rare – one Minnesota health official told CBS News kids have “about one in a million” chance of getting it – but it is crucial that any child displaying the signs get medical attention right away, the CDC says.

INSIDER spoke with pediatrician Dr. Natasha Burgert to learn more about AFM and its telltale symptoms. Here’s what you need to know.

The CDC says we still haven’t found a definitive cause of AFM

AFM affects the nervous system, particularly the spinal cord, causing muscles and reflexes across the body to become weak, the CDC explains. It’s often described as “polio-like” because polio also affects the nervous system, causing neurological changes, Burgert said.

AFM can happen in adults but tends to be more common in kids, she added.

It’s thought that AFM may be a communicable disease.

“Because there does seem to be a seasonal pattern and it does seem to happen in clusters, that makes us lean towards it being a communicable disease or an infectious disease,” Burgert said.

The CDC says it’s not clear why a small number of people who get these respiratory illnesses go on to develop AFM, while others recover.

Some experts in the field have pointed to one virus in particular – enterovirus D68, or EV-D68 – as a leading suspect behind many AFM cases, according to a report published October 29 by NBC News. The report noted that the 2014 nationwide uptick in AFM cases coincided with a widespread outbreak of EV-D68 illnesses.

But the CDC maintains that a definitive cause still hasn’t been found.

The agency has tested the spinal cord fluid of AFM patients to determine what’s behind the condition. Because AFM affects the spinal cord, finding a pathogen, or germ, in the spinal fluid would be “good evidence” that particular germ is the cause, the CDC website says.

There may be other explanations, too: AFM may be caused by an immune response to infection, rather than the infection itself, or there could be a genetic factor that makes some people more likely to get it, according to the CDC.

For now, the agency is continuing to investigate possible causes of AFM.

“That’s why it’s so scary,” Burgert said. “This is a potentially life-threatening illness and we don’t know the cause, and if we don’t know the cause, we can’t predict it.”

Anyone with AFM symptoms should see a doctor ASAP

Most people with AFM will experience sudden arm or leg weakness with a loss of reflexes and muscle tone, according to the CDC. Some people may also have facial drooping and weakness, drooping eyelids, difficulty swallowing, and slurred speech.

These signs may be a little harder to spot in young children, Burgert said, since they aren’t always able to articulate their symptoms.

“Since we know that this disease involves loss of muscle tone, I’m watching how [kids] are walking, how they’re drawing, how they’re using their utensils. For example, if they’re right-handed and now they’re only using their left arm, that’s going to be a huge red flag,” she said. “I would also look at their smile. A kid’s smile should always be symmetric.”

The most serious symptom of AFM is respiratory failure, the CDC says, which can happen when the muscles you use to breath become weakened. This is what makes AFM a potentially deadly disease, Burgert said.

But she stressed that breathing difficulties won’t be the first sign of AFM. The initial symptoms like limb and facial weakness do come on suddenly, but it takes some time for the disease to progress to the point where it affects breathing.

“Any astute parent with an appropriate level of concern will be able to reach hospitals and clinics for evaluation before that would happen,” Burgert said. “It’s not like they’re going to stop breathing in the middle of the night [because of AFM]. We just have to be attuned to the first signs.”

Finally, even though most AFM cases are preceded by a respiratory infection or fever, that doesn’t mean every kid who gets temporarily sick will end up with AFM.

“It’s important to know that this is still very rare and that the vast, vast majority of kids are going to have these normal cold and flu viruses with no complications,” she said.

There’s no specific treatment, but people with AFM can get supportive care in the hospital

“It’s not something you can go to a hospital and get a medicine for right now,” Burgert said.

Instead, she explained, people with AFM get supportive care in the hospital. Doctors will monitor their bodily functions, for example, and make sure patients who need breathing assistance get it. This care continues until the body starts to fight the infection on its own, Burgert said. After that, it’s a matter of recovery.

Since AFM attacks the neurons in the spinal cord, affecting neurological function, recovery from the condition is an attempt to regain that compromised function, Burgert explained.

“For kids that’s less of a worry,” she said. “Kids are so malleable, and kids with amazing neurological deficits can still regain that function as their brain and nervous system are growing and changing. In adults, when we have a neurological injury, it’s a lot harder to repair.”